There are numerous researches and inventions for improving semiconductor manufacturing technology nowadays. The photolithography process is necessary for patterning a semiconductor element and has become a bottleneck of the semiconductor process due to the continuously demanding of decreasing the semiconductor element size. That is to say, the semiconductor manufacturing technology will be hindered if the difficulties caused by implementing the photolithography technology cannot be overcome.
According to Rayleigh criterion, the minimum width, i.e. the resolution of the pattern, capable of being recognized by an optical system is directly proportional to the wavelength (λ) of a light and inversely proportional to the numerical aperture (NA). Therefore, in theory, both of the shorter wavelength of exposing light and bigger NA of lenses are conceivable to increase the resolution to obtain narrower width. However, the problems such as the shortening of the depth of focus (DOF) shall be considered. Currently, the Resolution Enhancement Technology (RET) commonly used in the photolithography process comprises Off-Axis Illumination (OAI), Phase Shift Mask (PSM), and Optical Proximity Correction (OPC).
A light passing through a mask will generate a diffraction effect when the line width is close to the wavelength of the light, and accumulation of the diffraction light will result in a serious distortion in the exposed patterns. The OPC technology compensates the distortion caused by diffraction effect and amends the patterns on the mask to enable the accumulated diffraction light to coincide with the required patterns and widths. However, the aforementioned technology still has drawbacks, such as increasing complexity of the mask and a high cost of the whole process.
Furthermore, when the development of the semiconductor manufacturing process comes to 45 nanometer or even a smaller size generation, an Extreme Ultra-Violet (EUV) may be adopted as a new light source, which is not necessary to use OPC or other RETs. However, high cost of the mask substrate as a glass with 40 layers of molybdenum and silicon and the capacity for producing a perfect mask substrate are heavy challenges.
In order to overcome the mentioned drawbacks in the prior art, a mask and the design method thereof are provided in the present preferred embodiment.